1. Field of the Invention
This invention relates to heat curable elastomer compositions. More particularly, this invention relates to storage stable, heat curable organosiloxane compositions in the form of a finely divided, free flowing powder. The compositions comprise a high consistency polydiorganosiloxane and a reinforcing silica filler. Optional ingredients include but are not limited to filler treating agents, non-reinforcing fillers, curing agents and/or curing catalysts.
2. Background Information
Organosiloxane compositions in the form of a free-flowing powder prepared from a high consistency "gum-type" polydiorganosiloxane and a reinforcing filler are described in a number of publications, including U.S. Pat. No. 3,824,208, which issued to Link and Scarbel on Jul. 16, 1974 and Japanese patent publication no. 2/102007, which was published on Apr. 13, 1990.
In accordance with the teaching of Link and Scarbel in the aforementioned patent, a powdered material is obtained by first reducing the particle size of the polydiorganosiloxane to within the range of from 0.1 micron to 25.4 mm. by suitable means and then mixing the particles with at least 15 parts by weight of a reinforcing filler. This operation is conducted at a temperature of from 0 to 100 degrees C. preferably from 20 to 60 degrees C. A second critical process parameter in addition to the temperature is the speed of the tip of the blade in the mixer or other device used to subdivide the polymer and blend it with the filler during preparation of the powdered material. In accordance with the teaching of the patent, this speed can generally range from 50 to 150 feet per second and is preferably from 70 to 150 feet per second.
Japanese patent publication no. 2/102007 teaches pelletizing a high consistency or "gel" type vinyl-containing polydiorganosiloxane having a viscosity of at least 1.times.10.sup.5 centipoise and then blending the resultant pellets with a filler. A processing aid is included to prevent a phenomenon referred to as "creping" or "crepe hardening". The resultant mixture is then fed into a mixing apparatus equipped with a high speed rotating blade that is capable of subjecting the mixture to high speed mechanical shearing. During this step of the process the mixture achieves the consistency of a free-flowing powder. The critical parameters for obtaining a free flowing powder are the speed of the blade tip and the residence time of the mixture in the mixing apparatus.
The Japanese patent publication also teaches that to achieve a homogeneous powdered material the temperature of the polymer-filler blend during shearing can be from 10.degree. to 100.degree. C. The temperature is preferably maintained near ambient by cooling the mixture as it is being converted to a powder.
Elastomers prepared using curable compositions prepared as described in the aforementioned United States patent and Japanese patent publication have a number of shortcomings, one of the more obvious being the presence of undesirable gel particles.
The present inventors discovered that formation of gel particles in finely divided organosiloxane compositions can be avoided or substantially reduced by using a set of process conditions for preparing the compositions that is beyond the limits and conditions described in the prior art. In addition to elimination of the gel particles the physical properties of cured elastomers prepared from the composition are substantially improved.
One objective of this invention is to provide an improved method for preparing curable organosiloxane compositions in the form of a flowable powder.
A second objective is to provide storage stable curable organosiloxane compositions comprising at least one high consistency or liquid polydiorganosiloxane and a reinforcing silica filler. The compositions can optionally contain a curing agent and/or a curing catalyst. The powder compositions are subsequently converted to high consistency organosiloxane compositions by fusing or "massing" of the powder particles and cured to yield gel free elastomers exhibiting excellent physical properties.